Oxygen sensors are used in a variety of applications in a variety of fields, including medicine, industry and scientific research. Commercially used oxygen sensors employ several different detection mechanisms, including electrochemical cells, and high temperature ionic conductors, such as solid state oxygen ion conduction based sensors, for instance, zirconia-based oxygen sensors in automobiles. However, these systems generally have relatively slow response times (because, for instance, gases must be transported through an electrolyte for detection to occur), or, as with the zirconia-based systems, require high operating temperatures.
Other oxygen sensors are optically based. Several devices have been proposed based on the luminescence quenching of various organic or inorganic materials which are typically dispersed in a polymer matrix. These materials permit the detection of oxygen concentration through a phenomenon known as photoluminescence "quenching".
In general terms, photoluminescence occurs when a material absorbs a photon of sufficient energy. The entity that absorbs the photon may be a discrete molecule, or a defect center in a solid-state material, for example, and is often referred to as a "carrier." When the photon has been absorbed, the carrier is moved into a high-energy, excited state. After a certain length of time, the carrier will relax back to its ground state. In so doing, it emits light. The lifetime of the excited state is usually on the order of nanoseconds to microseconds. The mechanism by which the carrier relaxes determines whether the photoluminescence is generally termed "fluorescence" or "phosphorescence." If the luminescence persists significantly after the excitation cause is removed, it is called phosphorescence; if it does not, it is called fluorescence.
If an oxygen molecule collides with a carrier while it is in its excited state, the oxygen molecule can absorb the carrier's excess energy and "quench" the photoluminescence. Quenching occurs when the oxygen molecule absorbs the energy and undergoes a triplet-to-singlet transition, while the carrier undergoes a nonradiative relaxation. The efficiency of the photoluminescence quenching is therefore determined by the number of collisions between the material containing the carrier, and oxygen molecules. Because the collision frequency of gases is determined by the concentration of quenching molecules present, the pressure and the temperature, the quenching efficiency for a given pressure and temperature, and consequently the photoluminescence intensity, will be determined by the concentration of oxygen in the atmosphere surrounding the material.
Various oxygen sensors based on the quenching of photoluminescent materials are known in the art. Examples of phosphorescent/photoluminescent materials used in oxygen sensors include a polymer immobilized metalloporphyrin (see Gewehr, P. M., Optical Oxygen Sensor Based on Phosphorescent Lifetime Quenching and Employing a Polymer Immobilized Metalloporphyrin Probe, Med. & Bio. Eng. & Compute 31, 2-10 (1993), transition-metal complexes (E. R. Carraway, et al., Photophysics and Photochemistry of Oxygen Sensors Based on Illuminescent Transition Metal Complexes, Anal. Chem. 63, 337-342 (1991), and dicyanoplatinum (II) complexes (Illuminescent dicyanoplatinum (II) complexes as sensors for the optical measurement of oxygen concentrations, W. W. Lee, et al., Anal. Chem. 5, 255-258 (1993), among others. See also, P. Hartmann, et al., Luminescence Quenching Behavior of an Oxygen Sensor Based on a Ru(II) Complex Dissolved in Polystyrene, Anal. Chem. 67, 88-93 (1995); W. Xu, et al., Oxygen Sensors Based on Luminescence Quenching: Interactions of Metal Complexes with the Polymer Substrates, Anal. Chem. 66, 4133-4141 (1994); A. E. Baron, et al., Submillisecond response times of oxygen-quenched luminescent coatings, Rev. Sci. Instrum. 64, 3394-3402 (1993); L. Sacksteder, Design of Oxygen Sensors Based on Quenching of Luminescent Metal Complexes: Effect of Ligand Size on Heterogeneity, Anal. Chem. 65, 3480-3483 (1993).
All of these materials have drawbacks. Many, particularly the organic materials, are susceptible to bleaching in a relatively short period of time. Bleaching adversely affects their photoluminescent character and thus shortens their useful lifetime. In addition, many of these materials are unstable at high temperatures and many have a slow response time, since in many cases oxygen must diffuse or otherwise migrate through a polymer (e.g., silicone) matrix in order to reach the photoluminescent material.
Accordingly, there is a need for improved oxygen sensors.